Many glycols are used for making unsaturated polyesters. Examples are ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, and neopentyl glycol. The degree of isomerization of maleate to fumarate largely depends on the glycol used. In general, the use of a primary glycol leads to a low degree of isomerization, while the use of a secondary glycol leads to a high degree of isomerization. For instance, the esterification of maleic anhydride with neopentyl glycol, ethylene glycol, and propylene glycol gives fumarate/maleate ratios of 50/50, 75/25, and 93/7, respectively. See Ind. Eng. Chem. Prod. Res. Dev. 3(3), 218 (1964). Although propylene glycol gives high fumarate content, it has lower reactivity toward condensation and the resultant unsaturated polyester often has a dark color and poor appearance.
2-Methyl-1,3-propanediol (MPD) is a relatively new glycol. It is an easily handled liquid, it has a high boiling point, and it has two primary hydroxyl groups for rapid condensation. However, like other primary glycols, MPD disadvantageously gives unsaturated polyesters having low fumarate/maleate ratios (60/40 to 70/30). Many efforts have been made to increase the fumarate content of unsaturated polyesters made from MPD. For instance, U.S. Pat. No. 6,492,487 teaches a process for making unsaturated polyesters from MPD that have fumarate/maleate ratios greater than 85/15. The process uses propylene glycol to boost the isomerization of maleate to fumarate in a late stage of the polymerization. Further, U.S. Pat. No. 6,555,623 teaches a process which comprises first reacting one equivalent of an aromatic diacid with about two equivalents of MPD to produce an ester diol and then reacting one equivalent of the ester diol with from about 1.1 to about 1.9 equivalents of maleic anhydride. The polyester produced has a fumarate/maleate ratio of 90/10 or greater.
Dicyclopentadiene (DCPD) modified polyester resins are also known. See for example U.S. Pat. Nos. 3,347,806, 4,029,848, 4,148,765, and 4,233,432. A general process of preparing DCPD modified polyester resins is first reacting a carboxylic acid or anhydride with DCPD in the presence of water to form a half ester and then reacting the half ester with glycols to produce the polyester.
DCPD is available in a number of grades. The low purity grades contain as much as 2 wt % of cyclopentadiene trimers. The “polyester grade” DCPD, which has less than 0.1 wt % of trimer, is commercially used to make unsaturated polyesters. Economically it would be desirable to use the less expensive low grade DCPD, but the use of the low grade DCPD yields polyester resins containing gel particles which plug spraying equipment and filters.
Efforts have been made to prepare gel particle free polyester resins using low grade DCPD. For instance, U.S. Pat. No. 6,515,071 teaches a process which involves forming a half ester of a low grade DCPD and a dicarboxylic acid in the presence of water by two stages: heating a mixture of DCPD, dicarboxylic acid and water at a relatively low temperature (<150° C.) and then raising the temperature up to about 220° C. The resulting half ester then reacts with glycols to form polyester. This process, however, is essentially the same as the above-discussed conventional process.
New processes for making DCPD modified polyesters are needed. Ideally, the unsaturated polyester is made from 2-methyl-1,3-propanediol, maleic anhydride, and a low grade DCPD and the unsaturated polyester has a high fumarate/maleate ratio.